1. Field of the Invention
The present invention relates to a sync generator unit and a field decision unit applied to a telecasting system or a video playback system, and particularly to a sync generator unit for generating an internal synchronizing signal containing synchronizing signal components synchronized with the synchronizing signal components of a video input signal such as a composite video signal for telecasting including the synchronizing signal components, and a field decision unit for making a field decision of the video input signal using the sync generator unit.
2. Description of Related Art
FIG. 5 is a block diagram showing a configuration of a conventional field decision unit. In FIG. 5, the reference numeral 1 designates a sync separation circuit for seating from a composite video signal Cv (video input signal) its horizontal and vertical synchronizing signal components and outputting them as the horizontal synchronizing signal Hs and vertical synchronizing signal Vs; 2 designates a phase comparator for receiving the horizontal synchronizing signal Hs and internal synchronizing signal Is, and for generating period correcting pulses Pc with a pulse width corresponding to the input timing difference between the synchronizing signal components contained in the two inputs; 6 designates a lowpass filter (LPF) for smoothing the period correcting pulses Pc and producing a level signal corresponding to their pulse width; 7 designates a voltage controlled oscillator for generating a signal with a frequency corresponding to the level signal, 8 designates a frequency divider for dividing the frequency signal output from the voltage controlled oscillator 7 and for outputting the internal synchronizing signal Is; 5 designates a field decision circuit which receives the internal synchronizing signal Is and vertical synchronizing signal Vs, and makes a field decision based on the two inputs; and 4 designates an output controller for producing an output halt signal Ph for suspending the output of the period correcting pulses Pc from the phase comparator 2.
Next, the structure of the composite video signal will be described of the NTSC (National Television System Committee) color system widely employed in the telecasting. Since the composite video signal draws a display screen with 525 horizontal scanning lines, its period corresponds to 525 horizontal synchronization cycles. In addition, since each screen is displayed using interlace scanning, a pulse train for the vertical synchronization is superimposed at every 262.5 scanning line intervals. More specifically, each field consisting of 262.5 scanning lines includes, in its initial nine horizontal synchronization cycles, equalizing pulses superimposed thereon with a period of half that of the horizontal synchronizing signal. In particular, the phase of the equalizing pulses are inverted (that is, shifted by 180 degrees) during three cycles from fourth to sixth horizontal synchronization cycle.
Next, the operation of the conventional field decision unit will be described.
Receiving the composite video signal Cv, the sync separation circuit 1 outputs the horizontal synchronizing signal Hs at every horizontal synchronization cycle. The phase comparator 2, comparing the synchronizing signal component of the horizontal synchronizing signal Hs and that of the internal synchronizing signal Is, makes a decision of the input timings of the two synchronizing signal components, and outputs the period correcting pulses Pc with a pulse width corresponding to the input timing difference. The period correcting pulses Pc vary the level of the level signal output from the lowpass filter 6, the oscillation frequency of the voltage controlled oscillator 7, and then the frequency of the internal synchronizing signal Is output from the frequency divider 8. Thus, the phase of the synchronizing signal component of the internal synchronizing signal Is varies such that it matches the phase of the synchronizing signal component of the horizontal synchronizing signal Hs, resulting in the synchronization of the two synchronizing signal components.
In parallel with this, the sync separation circuit 1 outputs the vertical synchronizing signal Vs at every field interval of the composite video signal Cv. It is generated on the basis of the input timings of the equalizing pulses with their phase inverted in the fields. Considering the phase of the vertical synchronizing signal Vs with respect to that of the synchronizing signal component of the internal synchronizing signal Is, the field decision circuit 5 makes a field decision based on the phase relation. More specifically, as is apparently seen by comparing FIGS. 6A and 6B, when the synchronizing cycle of the internal synchronizing signal Is in completely in synchronization with the horizontal synchronizing cycle, the phase of the vertical synchronizing pulse with respect to that of the synchronizing signal component of the internal synchronizing signal Is in the first field as indicated by C of FIG. 6A is shifted from that in the second field as indicated by D by an amount of half the synchronizing cycle because each field consists of 262.5 horizontal synchronizing cycles. Thus, the field decision unit identifies the case A of FIG. 6A, in which the synchronizing signal component of the internal synchronizing signal Is arrives immediately after the vertical synchronizing pulse, as the first field, and the case B of FIG. 6B, in which it arrives just before the vertical synchronizing pulse, as the second field.
The output halt signal Ph supplied from the output controller 4 to the phase comparator 2 as shown in FIG. 5 controls the phase comparator 2 so that it suppresses the period correcting pulses Pc that would allow the synchronization of the internal synchronizing signal Is with the equalizing pulses superimposed on the middle positions of the horizontal synchronizing cycles.
A telecasting receiving system or a video playback system incorporating such a field decision unit generates ramp waves based on the horizontal synchronizing signal Hs and vertical synchronizing signal Vs, and supplies a display like a cathode-ray tube (CRT) with a voltage based on the composite video signal Cv while driving it with the ramp waves, thus displaying restored images by scanning the display.
With such a configuration, the conventional field decision unit has a problem in that it suffers from erroneous phase deviation of the internal synchronizing signal Is from the composite video signal Cv by an amount of half the synchronizing cycle due to noise superimposed on the composite video signal Cv and the like.
This will be described in more detail.
Such a malfunction can be prevented by producing the output halt pulses Ph in almost allover the synchronizing cycles to effectively suppress all types of noise. This, however, will present a new problem. For example, when the internal synchronizing signal Is has not yet synchronized with the composite video signal Cv as in the case of starting the system or switching the composite video signal Cv, it is difficult to identify the synchronizing cycle of the composite video signal Cv, and there would be a possibility that the synchronization cannot be established in the worst case. Therefore, the output duration of the output halt pulses Ph is set rather short based on such a practical observation.
With respect to this, the composite video signal Cv includes an interval (called the equalizing pulse interval from now on) during which the equalizing pulses occur at every half the synchronizing cycle interval. If there is some incoming noise during the equalizing pulse interval, that is, from the first to ninth synchronizing cycle of each field, the phase comparator 2 and sync separation circuit 1 will operate outside the output halt period such that they try to establish synchronization with the noise because they confuse it with the synchronizing signal component. Since the output period of the output halt pulses Ph is set rather short as described above, the pulse width of the period correcting pulse Pc due to the confusion can become broad, thereby varying the synchronizing signal component of the internal synchronizing signal Is by a large amount. This will result in the synchronization of the synchronizing signal component of the internal synchronizing signal Is with the equalizing pulses at the middle of the synchronizing cycle because the conventional output halt period is set rather short. As a result, the field decision made after these equalizing pulses will identify the first field as the second field, or vice versa, because of the phase deviation of the internal synchronizing signal Is by an amount of half the synchronizing cycle with respect to the composite video signal Cv.
In view of this, a field decision unit is proposed that continues the output of the output halt pulses Ph throughout the equalizing pulse interval as the field decision unit disclosed in Japanese patent application laid-open No. 5-56304/1993.
However, this technique cannot fully prevent the erroneous synchronization with the equalizing pulses. For example, when the incoming noise occurs just before the equalizing pulse interval, the internal synchronizing signal Is can be synchronized with the equalizing pulses at the middle of the synchronizing cycle because of the synchronization control based on the noise.
In addition, the foregoing technique does not ensure the correct synchronization in the equalizing pulse interval, and even minimal differences of synchronization can cause, when accumulated, the deviation in the field decision timing, resulting in erroneous field decision or synchronizing cycle identification. This will further presents a secondary problem in that once the synchronization is lost, it cannot be recovered in a short time even if the synchronization control is restarted immediately.